Inorganic sponge blowing agents



blowing sponges.

Patented Mar. 6, 1951 INORGANIC SPONGE BLOWING AGENTS Arthur A. Baum,Wilmington, Del., assignor to E. I. du Pont de Nemours & Company,Wilmington, Del., a corporation of Delaware No Drawing. ApplicationNovember 27, 1948, Serial No. 62,411

3 Claims.

This invention relates to the preparation of rubber spongematerial, andmore particularly to the preparation of stable dispersions of inorganicsponge blowing agents and their use in the manufacture of rubber-likesponge material.

Rubber and other elastomeric materials can be converted to sponge formby incorporating into the elastomeric mixture a sponge blowing agentprior to vulcanization. On heating to vulcanize the mixture, the spongeblowing agent gives off a gas to form the sponge. Many variations ofthis process are described in the literature in which, for example, themixture may be partially vulcanized under high pressure to avoid theexpansion of the gas, then expanded to a sponge, and finally vulcanizedcompletely in the expanded state, or a mold may be used of such a sizethat the expanding mixture is blown and vulcanized in one step but-under conditions such. that the sponge is under pressure at the end ofthe vulcanization.

In general, two types of agents are used for The inorganic type asexemplified by sodium bicarbonate, which is presently used, tends togive a sponge having large, uneven pores. Large amounts of the carbonateare needed, and, as a consequence, the resulting sponge contains amountsof electrolytes which in some cases is found to be undesirable. Theorganic type of sponge blowing agent, such as diazoamino benzene, ismuch more efficient on a weight basis and gives a sponge having fine,uniform pores. However, this type of blowing agent is considerably moreexpensive than the inorganic type.

It is therefore an ob ect of this invention to prepare inorganic pongeblowing agents of an impro ed form. It is a further ob ect of theinvention to produce sodi m or ammonium carbonates or bicarbonates as adispersion in oils or waxes which are highly effective in blowing of elatomer sponge and which produce sponge of finer texture and greateruniformity than is ordinarily possible to obtain with the inorganicsponge blowing agents now available. A still further object of theinvention is to provide a process for the manufacture of elastomericmaterial in sponge form of greatly improved physical character stics inwhich inorganic sponge blowing agents are employed.

In the manufacture of elastomeric (such as rubber of synthetic rubber)sponge, the inorganic sponge blowing agents, ordinarily sodium orammonium carbonates or bicarbonates, are employed in a dry powder form.The size of the small crystals or particles, even when these materialsare finely powdered, is relatively large even though reduced to a pointwhere 60% or 80% of the material will pass through a 100 mesh sieve.Even extremely fine powders, such as those reduced to where they willpass through a standard 200' mesh sieve; will have a weight 'avera'ge'particle diameter of approximately 60 microns. Such material reactsrelatively slowly during the sponge blowing operation, and producescoarse, uneven gas cells in the sponge and, as pointed out above,relatively large amounts of this material are required.

I have found that, when sodium or ammonium carbonates or bicarbonatesare ground in oil in the presence of an oil soluble dispersing agent,stable di persions in the hydrocarbon oil can be produced in which theweight average particle diameter of the carbonate or bicarbonate isbelow 10 microns. When this stable dispersion of the carbonate orbicarbonate in oil is employed as a sponge blowing agent, it operatesmore efficiently than the customary dry powder that is ordinarily used,and produces a sponge having much finer pores and of greater uniformity.

It has been found that the use of an oil soluble dispers ng agent isnecessary if the crystals of the carbonate or bicarbonate are to bebroken down during the grinding in the hydrocarbon oil. The gr ndingshou d be carried out to a point whereby upon microscopic inspection itis found that not over 5% of the total weight of the partic es of thecarbonate or bicarbonate are more than 15 microns in their greatestdimension. U"ually, it will be found that, when the material has beenground sufiiciently long that the major porton has been reduced to aparticle size of not greater than from 5 to 10 microns, there will beless than 1% of the material of a particle size larger than 15 micronsin its greatest dimension. In general, during the grinding operation,particularly where it is carried out in a ball m ll, a stable oildispersion of the carbonate or bicarbonate is obtained when the weightavera e particle diameter of 15 microns or less is attained. By weightaverage diameter, we refer to the sum of the products of the wei ht ofthe part c es multipled by the diameter of the particles, divided by thetotal weight of the sample. When this degree of fineness is attained,however, it will be found upon microscopic examination that less than 5%of the total weight of the carbonate or bicarbonate will be in particlesgreater than 15 microns in their greatest dimension, and ordinarily theamount of part cles of 15 microns will be much less than 5% of the totalweight.

Any neutral liquid hydrocarbon, or solid hydrocarbon which is liquidunder the conditions of grind ng, may be employed in the preparation ofthese d spersions. It is preferable that the hydrocarbon employed shouldhave a flash point above F. and a melting point below 200 F. The use ofthe solid or more viscous hydrocarbons ofier the advantage that theresulting products 3 have high storage stability, although their usepresents some added difficulties in carrying out the grinding operation.For obvious reasons, the use of a very volatile hydrocarbon is notdesirable.

Any type of mill may be employed which will reduce the particles to thesmall size desired. The usual ball mill is found to be entirelysatisfactory.

In the grinding operation, a wide variety of oil soluble dispersingagents may be used to assist in grinding to a small particle size. Ingeneral, these agents should contain a hydrocarbon chain suflicientlylong to impart oil solubility, and, in addition, a strongly polar groupto impart surface active properties. The amount of this agent to be useddepends upon the activity of the particular agent. The followingexamples illustrate that 1%, based on the oil phase. is adequate in allcases. With more effective agents, this amount may be reduced to atleast 0.1% on the oil phase, and in many cases still lower.

The preferred amount of dispersing agent to be employed will be between0.5% and 2%. based on the amount of oil employed, although largeramounts up to 10% or even more may be used. While any of the oil solubledispersing agents may be used to disperse the carbonate or bicarbonatein the hydrocarbon medium, the use of lecithin, zinc naphthenate orsodium petroleum sulfonate give the best results. (See Ind. 81 Eng.Chem, vol. 40, page 890, May 1948.)

In general, it is found desirable to prepare the dispersions having asolids content of from 40% to 60%. By conducting the grinding operationstepwise, it is possible to increase the solids content to at least 70%.Since these blowing agents are eflective in such small amounts, it isfound that for practical purposes the 50% dispersion of the carbonate orbicarbonate in the hydrocarbon medium is satisfactory. There is noparticular advantage in preparing a dispersion having low solidscontents, since it merely adds to the amount of oil or solid hydrocarbonthat must be incorporated in the elastomer.

The following examples are given to illustrate the invention. The partsused are by weight, unless otherwise specified.

Example 1 A mixture of 100 parts of sodium bicarbonate, 100 part ofwhite paraflin oil Nujol and 1 part of lecithin was ground in a ballmill containing flint balls for 8 days. The resulting product was athin, white, oily suspension which showed no separation of oil uponstanding for several days. A photomicrograph of the material showed itto have an average particle diameter of under 10 microns, and that lessthan 5% of the weight of the material was in particles which were morethan 15 microns in their greatest dimension.

When a similar mixture of 100 partsof sodium bicarbonate and 100 partsof white paraffin oil Nujol" was ground under identical conditionsexcept no lecithin was present, the product obtained after 8 daysgrinding was a pasty mass which showed oil separation after standing afew days. A photomicrograph of the material showed it to have a weightaverage diameter of at least 40 microns. Substantially all of the sodiumbicarbonate in the mass consisted of particles larger than 30 microns intheir greatest dimension.

I 100 parts of mineral oil Circo light process oil,"

and 2 parts of zinc naphthenate "Nuocide zinc 12 was milled with gravel(pea size) for 3 days. The product was a stable, light yellow dispersion.A photomicrograph of this dispersion showed only a few particles, ifany, larger than 5 microns in their greatest dimension.

Example 3 In a ball mill half full of one-half inch steel balls, wasplaced a mixture of parts of sodium bicarbonate, 100 parts oi. mineraloil "Circo light process oil, and 2 parts of lecithin. After rolling for3 days, a smooth, stable dispersion having a weight average particlesize under 10 microns was obtained. A photomicrograph of this productshowed substantially all particles under 10 microns in their greatestdimension.

Example 4 A mixture of 100 parts of ammonium carbonate, 100 parts ofmineral oil and 2 parts of lecithin was milled in the same manner asExample 2. The product was a stable dispersion containing particleshaving a weight average diameter under 10 microns. Only a few, if any,of the particles were larger than 5 microns in their greatest dimension.

Example 5 Mixtures of 100 parts of sodium carbonate and 100 parts ofmineral oil Circo light process oil" were made up with the followingadditives:

A-None, control B-One part of lecithin C-One part of zinc naphthenateDOne part of sodium petroleum sulfonate.

These mixes were rolled in mill half full of ravel (pea size) for fivedays. At the conclusion, A was a thick paste containing coarse particlessimilar to those of the control sample of Example 1 (paragraph 2), whileB, C and D were stable dispersions having average particle diametersbelow 10 microns. Only a few particles in each of these products wereover 10 microns in their greatest dimension.

Example 6 dispersions, while the one-containing no lecithin was a thick,coarse paste.

' Example 7 Mixtures of 100 parts of sodium bicarbonate and 100 parts ofmineral oil fCirco light process oil were milled with gravel as inExample 2 with one part of the following additives:

Calcium petroleum sulfonate Dianrvl amine Dioctyl phthalate Glyceryldioleate Lauryl amine stearate Lead naphthenate Octadecyl nitrile Oleicacid Oleyl amine Sodium dioctyl sulfosuccinate Sodium petroleumsulfonate Stearic acid Tetradecane amide.

In each case, the product was a uniform dispersion containing particleshaving a weight Example 8 the density and four times the volume increaseobtained with 5 parts directly. In each case. the carbonate dispersed in011 according to the present invention gave sponge of fine even tex- 5ture. while thesponge made by the prior art sodium blcarboliate was{Timed as in Example method of direct addition of carbonate was coarse 1with equal 3111191111 011 Circo light and irregular. Similarly, theother carbonates of process 011 containing 0.1%, 0.25%, 0.5%, 1.0%.Examples 4, 5 and 6, etc., or the dispersions 2.0% and 4.0% of zincnaphthen e- In each tained as described in the other examples, givecase, a stable dispersion was obtained with par- 10 elastomer spongeshaving the fine even texture ticles hav ng a weight average diameter ofless m t t in this example and Show simmer than 10 microns. Less than byweight of the fficiency in operation. particles were over microns intheir greatest To insure a unifo m fine pore sponge t dlmensionorganicblowing agents of this invention should x p e 9 have a weight averageparticle diameter not To a ball mill half full 01. flint balls was addedgreater than 15 microns and not more than 5% 100 parts of sodiumbicarbonate, 100 parts of of the total weight of the particles should bemore petrolatum (liquid at from 47 to 50 C.) and one than 15 microns intheir greatest dlmensionpart of lecithin. The mill was rotated underWhere Petrolatum x i us d as the hydrojnfra red light fo 14 days, Thelamps were carbon medium, the 15 micron size has been found justed tokeep the charge molten. The product to be f Where a uquld hydrocarbonwas a smooth homogeneous a te containing medium 15 elnployed, is usuallydesirable to particles having a weight average diameter of not have thee ht average particle diameter not more than 5 microns. There weresubstantially greater than 10 mlcl'ons as to msPre stablhty no particlesin the mass over 10 microns in their of the dispersion over extendedperiods of time greatest dimension and under adverse conditions.

- As illustrated by the examples, the use of the Example 10 very finelydispersed carbonate or bicarbonate In a manner similar to Example 9, amixture of reselts i much greater i n y an a r 100 parts of sodiumbicarbonate. 100 parts of id rat f bl w in the p pa n of lasparafin wax(liquid at 55 C.) and 2 parts of tomer sponges. Since much smalleramounts of lecithin was milled with gravel (pea size) under F spongeblowing agent are n e ed, e resul infra-red light. The productsolidified on 0001- g sponge will c n in l s l r ly whi h in mg.Microscopic examination of a solution of many (Eases of importancethismaterial indicated a weight average particle I 01mm: diameterconsiderably under 10 microns Qnly A sponge Plowing agent comprising ahydroa few particles (less than 1% by weight) were CFLIbOII dlsperslon oa compound of the class conover 5 microns in their greatest dimensionsisting of ammomum carbonate, ammonium b1- E l 11 carbonate, sodiumcarbonate and sodium bicarxamp e 40 bonate, and an oil solubledispersing agent, said Sponge mixes of various elastomers were madecompo having n average Particle diameter up on a rubber mill. Thesecontained, as blowof not over 15 microns and less than 5% of such ingagents, ordinary sodium bicarbonate and a compound y Weight being inparticles larger 50% dispersion of sodium bicarbonate in oil than 15microns in their greatest dimension, said (made as in Example 3 exceptthat 1 part of hydrocarbon being one which hasamelting point lecithin,instead of 2 parts, was used) having a below 200 F. and a flash pointover 80 F. weight average particle diameter substantially 2. A spongeblowing agent comprising a hydroless than 10 microns. Very few, if any,particles carbon dispersion of a c mpound of the class conin thedispersion were over 15 microns in their sisting of ammonium carbonate,ammonium bigreatest dimension. These mixes were then vulcarbonate,sodium carbonate and sodium bicarcanized and blown in one operation in amold bonate containing from 0.1% to 10%, based on having more thansuflicient room for any exthe hydrocarbon, of lecithin, said compoundhavpansion (a so-called free blow). The results in an average P r le dimeter of not over 15 of these tests are given in Table I. microns andless than 5% of such compound by TABLE I Polychloroprene (GR-M) 100.0100.0 Butadienestyreneinterpolymer (GR-S)- 100.0 100.0 Rubber (PaleCrepe) 100.0 100.0 36% xylyl mercaptan solutiom. 0. 0. 5 zinc xylylmercaptide solut 2.0 2.0 Extra light calcined magnesia.-- 4. 0 4. 0Phenyl alpha naphthyiamine- 2.0 2.0 1.0 1.0 1.0 1.0 Zinc oxide 5.0 5.05.0 5.0 5.0 5.0 std-me acid 3.0 3.0 10.0 10.0 Petrolatum 3.0 3.0 3.0 3.03.0 3.0 ulfur 3.0 3.0 3.5 3.5 MFT carbon black 50.0 50.0 Whitin 50.050.0 0.3 0.19 0.10 Circo"light process 00.. a 15:0 15:0 20:0 0 10:0 10:0Sodium bicarbonate 10.0 5.0 10.0 50% sodium bicarbonate disoersioninoi]4.0 5.0 4.0

Cure: 30 minutes at 307 F. Density of sponge, gJcc 0.223 0.210 0.7140.353 0.347 0.291 Volume increase on blowing, per cent 492 474 57 218240 307 It will be noted that only 2 parts of bicarbonate ground in oilgave the same or better blowing (lower density, greater volume increase)than 10 parts of the carbonate put directly into the rubber, and that2.5 parts ground in oil gave half weight being in particles larger than15 microns in their greatest dimension, said hydrocarbon being one whichhas a melting point below 200 F. and a flash point over F.

3. In the process of producing sponge-like material from elastomers otthe class consisting of natural rubber and synthetic rubber-likematerials in which an inorganic sponge blowing agent is employed, thesteps which comprise incorporating into the elastomeric material ahydrocarbon dispersion of a compound of the class consisting of ammoniumcarbonate, ammonium bicarbonate, sodium carbonate and sodiumbicarbonate, which compound has an average particle diameter of not over15 microns and less than 5% of such compound by weight being inparticles larger than 15 microns in their greatest dimension, andvulcanizing the resulting mixture, said hydrocarbon being one which hasa melting point below 200 F. and a. flash point over 80 F.

ARTHUR A. BAUM.

REFERENCES CITED UNITED STATES PATENTS Name Date Number North Oct. 12,1926 Number Name Date 1,912,591 Olin June 6, 1933 2,079,051 Sullivan eta1 May 4, 1937 2,201,064 Thurman May 14, 1940 2,209,451 Geyer July 30,1940 2,299,593 Roberts et a1 Oct. 20, 1942 2,466,027 Horney et al Apr.5, 1949 FOREIGN PATENTS 10 Number Country Date 464,174 Great BritainApr. 13, 1937 OTHER REFERENCES Du-Pont Sponge Rubber Compounding, pp.1-5,

15 Report No. 38-6, June 1938, pub. by Du Pont'Rub- 20 her 1935, byScientific Sec. Natl Paint, Var., and

Lac. Assn.

1. A SPONGE BLOWING AGENT COMPRISING A HYDROCARBON DISPERSION OF ACOMPOUND OF THE CLASS CONSISTING OF AMMONIUM CARBONATE, AMMONIUMBICARBONATE, SODIUM CARBONATE AND SODIUM BICARBONATE, AND AN OIL SOLUBLEDISPERSING AGENT, SAID COMPOUND HAVING AN AVERAGE PARTICLE DIAMETER OFNOT OVER 15 MICRONS AND LESS THAN 5% OF SUCH COMPOUND BY WEIGHT BEING INPARTICLES LARGER THAN 15 MICRONS IN THEIR GREATEST DIMENSION, SAIDHYDROCARBON BEING ONE WHICH HAS A MELTING POINT BELOW 200* F. AND AFLASH POINT OVER 80* F.